Drilling wells for various purposes is well-known. Such wells may be drilled for geothermal purposes, to produce hydrocarbons (e.g., oil and gas), to produce water, and so on. Well depth may range from a few thousand feet to 25,000 feet or more.
Tools for measurement of properties of the well or the formation may be conveyed downhole. Tools having an acoustic device for measuring acoustic velocities of subsurface formations (during or after drilling of the wellbores) are well known. These tools may be incorporated in drilling systems (e.g., “logging-while-drilling” (‘LWD’) or “measurement-while-drilling” (‘MWD’) systems) or an after-drilling wireline logging system and used in determining formation characteristics and the location of fractures or other boundaries with respect to the tool. Tools for imaging of acoustically reflective boundaries using directional acoustic sources are also known.